The continent-arc collision in northern Papua New Guinea

    Recent geological mapping in the north coast ranges of Papua New Guinea has recognised a Paleogene island arc. This arc is believed to have faced southward, and formed at the northeastern boundary of the Indo-Australian plate. The arc collided with continental crust of the Indo-Australian plate to the south; collision is thought to have occurred first in the west in the Early Miocene and to have progressed eastwards. Crustal shortening on collision resulted in foreland-type folding and thrusting at the continental margin, emplacement of ophiolite allochthons from the arc-trench gap at the collision zone, and uplift and fracturing of the accreted arc. Post-collision plate adjustments are thought to include extensive trans- current faulting about the former plate boundary, southward thrusting of part of the arc (Finisterre-Huon block), and extensive faulting in a complex linear zone extending from south of Manus Island through New Ireland and the Solomon Islands. Present-day interaction between the Indo-Australian and Pacific plates is spread over a wide zone in which at least two minor plates are involved.
    Despite previous claims, plots of the most accurately located earthquake foci define a northward-dipping seismic zone beneath the Late Cainozoic volcanoes at the southern margin of the Bismarck Sea. We find no evidence to substantiate a reversal of arc polarity at any time after the Mid-Tertiary collision. Mid to Late Cainozoic magmatism in central Papua New Guinea appears to have been triggered by uplift inducing partial melting of mantle modified by Cretaceous subduction. The present-day northward-dipping seismic zone is believed to be a vestige of the Early Tertiary subduction zone; the hanging slab is now slowly sinking and equilibrating with the mantle. If northern New Guinea can be considered to be the type example of a continent/island-arc collision then reversal of arc polarity may not be a necessary consequence of such collisions.